How does your tablet know that you are tilting it to take a turn in your racing game? How does your smartphone know which option you have selected when you tap it? How does a pacemaker vary the heartbeat in relation to the activity of the body? The answer: MEMS or MicroElectroMechanical Systems.
They are microscale devices, measuring between 1 micrometre to 1 millimetre, having both electrical and mechanical components. They may consist of a microprocessor, analog to digital converters and signal amplifiers along with mechanical components that interact with the outside world such as microsensors used for measuring acceleration, rotation, sound, impact, vibration and so on.
Well before the technology to make miniaturised machines became a reality, the potential was highlighted by Richard Feynman in his famous 1959 lecture ‘There’s plenty of room at the bottom’. However, MEMS became practical only when the microscale structures could be fabricated using modified semiconductor device fabrication technologies.
Inertial MEMS sensors are MEMS devices that make the examples quoted above possible. They convert a physical phenomenon, such as acceleration, vibration, rotation, tilt or shock, into a measurable electrical signal. The most typical inertial MEMS sensors are the accelerometer, which measures acceleration or translational motion, and the gyroscope, which senses rotation. MEMS technology has reduced size and cost of these sensors dramatically compared to conventional mechanical accelerometers or gyroscopes. This led to MEMS being used in applications that were not possible earlier. For example, the early 1990s saw Analog Device pioneering inertial MEMS being used in automotive airbags.
Capturing clear and crisp images, especially without a tripod, is a challenge even for professional photographers. When a photographer’s hands are not steady, the camera lens rotates resulting in a blurry image, especially at slow shutter speeds and in low light conditions. A two axis gyroscope is used to measure the movement of the camera, and a microcontroller directs that signal to small linear motors that move the image sensor, compensating for the camera motion. Light strikes the pixels of the image sensor as though the camera was not shaking,
Motion is already a “must have” function in game consoles, as demonstrated in the Nintendo Wii and Sony PlayStation 3 controllers, and in cellphones where motion processing is used for display orientation, gaming and user interface functionality. Accelerometers and gyroscopes are used in a complementary fashion to identify different types of motion — gravity, linear motion and rotational motion and capture the type of movement, such as striking a tennis ball when playing a virtual tennis game on your gaming console.
The dependence on GPS-based navigation systems is increasing by the day. GPS devices provide latitude, longitude and some may calculate altitude information, which can be used in navigation. But GPS data may not be continuously available due to possibility of signal blockage and other factors. In the event of GPS signal dropout, MEMS sensors (accelerometer and yaw rate gyroscope) act as a ‘flywheel’ to maintain vehicle orientation and help ensure accurate vehicle positioning. Similarly, in urban combat situations, positioning of each soldier may not be possible with GPS alone. MEMS sensors are used to detect and pinpoint accurate information of soldier position based on a GPS reference.
Modern automobiles have several sensors. However, the two most important applications are airbags and Electronic Stability Control (ESC). High-g accelerometers are installed to detect the collision and actuate an airbag.
While for ESC, a gyroscope compares the driver’s intended direction, which is determined from steering wheel position and if there is any difference between intended and actual direction, and ESC system will apply braking to a wheel and bring the car back in the control of the driver.
Inertial motion sensors enable a new portable surgical navigation system that guides femoral alignment during knee surgery. This system allows an orthopaedic surgeon to quickly determine the centre of rotation of the patient’s femur and calculate the precise angles to cut the bone in knee replacement surgery, providing alignment precision comparable to the more expensive camera-based navigation systems.
Similarly, the inertial sensors can be used further to optimise battery operation in pacemakers, real-time motion capture, for anti-theft alarms in cars, fall-detection or impact detection for athletes, gesture detection and so on.
(The authors are Vikas Choudhary, Senior Engineering Manager, ADI, and Deva Phanindra Kumar, Senior Engineer for the MEMS and Sensor Technology Group at ADI)